There are many optical applications where it is desirable to have one optical objective form several images on different focal planes so that several separate detectors may view the same scene element. Examples of such applications are color TV cameras which use three vidicon tubes to record the scene in three colors and photographic cameras used in converting a color scene into three color images for color printing presses. Another application of particular importance is with respect to multi-spectral linear array mapping cameras which include several linear arrays of detectors located in the focal plane of a lens situated, for example, in an orbiting satellite. If the several linear arrays are parallel and if the direction of motion of the satellite is perpendicular to the arrays, a swath type of image will be generated as each array is swept across the earth in what is known as a pushbroom scan mode. Since the several linear arrays are not coincident, each array will see a point on the earth at a different delayed time. This time delay causes problems if the satellite rolls, pitches or yaws in an unpredictable manner between the passage of the several arrays. Accordingly, it becomes impossible to superimpose data of the arrays in a simple time delay process for generating an image. Also, if there is field dependent distortion in the objective, it is difficult to reconstruct the image without complicated computer models of the distortion. Further, if the scene on earth is viewed obliquely, it is extremely difficult to reconstruct the scene due to the fact that each detector array sees the scene at a slightly different angle.
For these reasons, it is normally desirable to place a selected number of beam splitters behind an objective lens and divide the output of the lens into several beams of light, each of which impinges on its own focal plane. This process shares the energy from the lens among the several focal planes, but uses only one common objective lens and permits all the linear arrays to be conjugate to a common line to the scene of view.
A serious limitation of this apparently simple solution exists when a plate or block type beamsplitter is placed in a converging ray light beam due to the fact that extremely undesirable optical aberrations are produced by the beamsplitter. These aberrations are of the well known chromatic and Seidel type. These aberrations can be compensated for when a block type beamsplitter is used if compensating aberrations are built into the lens, such as by designing opposite sign aberrations into the image forming section of the systems so that the sum of the compensating aberrations and the beamsplitting aberrations equals zero. However, the compensation process requires several additional optical elements and if the objective is large compared to the detector array, the complexity and cost of the components may be extremely high. Further, if the system is of a large aperture type, i.e. on the order of several feet in diameter, refractive elements can become prohibitively heavy requiring use, for example, of a reflective objective. Furthermore, with a reflective objective, a problem of eliminating aberrations becomes even more difficult because the designer has relatively little freedom to compensate for the chromatic aberrations in the beamsplitters.
One known method of eliminating beamsplitter aberrations is through use of pellicle type beamsplitters incorporating microscopically thin membranes stretched over a frame and coated with an evaporated beamsplitter layer. While pellicles effectively eliminate the optical problems, they are extremely fragile and are very prone to vibrations in a noisy environment. Another known method includes the use of a two element relay lens designed so that the rays are collimated between the elements. Beamsplitters are located between the two relay lens elements in the collimated region. Since the rays are not converging, no aberrations occur. The disadvantage of this method is that for each split, two reimaging lenses and one collimating lens must be used. Further, the problem of designing the relay lens becomes more difficult than that of designing the original objective if wide fields of view are required.